Line pipes used for transporting crude oil or natural gas, containing hydrogen sulfide, are required to have what is called “sour-resistance” including HIC resistance and resistance to stress corrosion crack (SCC resistance), adding to strength, toughness, and weldability. The phenomenon of hydrogen-induced cracking (HIC) of steel is based on a process in which hydrogen ions generated by corrosion reaction are adsorbed on the surface of steel, penetrate into steel as atomic hydrogen, and diffuse and accumulate around non-metallic inclusions such as MnS and hard second phase of steel, thus triggering crack initiation by an increase in internal pressure.
To prevent that type of HIC, JP-A-54-110119, (the term “JP-A” referred herein signifies the “Japanese Patent Laid-Open No.”), discloses a method for manufacturing a steel for line pipe having excellent HIC resistance by adding an adequate quantity of Ca and Ce depending on the amount of S to suppress generation of acicular MnS and to produce finely dispersed spherical MnS, giving less stress concentration, and thus suppressing initiation and propagation of crack. JP-A-61-60866 and JP-A-61-165207 disclose steels, having excellent HIC resistance, with suppressed formation of martensite-austenite constituent which acts as an initiation site of crack in center segregation zone, and with suppressed formation of hard phase such as martensite or bainite which acts as a propagation route of crack, through reduction in the amount of elements (C, Mn, P, and the like) having strong segregation tendency, soaking treatment in a slab-heating stage, and accelerated cooling during transformation in a cooling stage after hot rolling. For an X80 grade high strength steel plate having excellent HIC resistance, JP-A-5-9575, JP-A-5-271766, and JP-A-7-173536 disclose a plate manufacturing method of suppressing center segregation by reducing C and Mn content, controlling morphology of inclusions by reducing S content and adding Ca, and compensating decrease in strength caused by the reduced C and Mn content by adding Cr, Ni, and the like and by applying accelerated cooling after hot rolling.
The above-described methods for improving HIC resistance, however, deal mainly with suppression of center segregation. On the other hand, for high strength steel plates of API X65 or higher grade which are often manufactured by accelerated cooling or direct quenching, the near surface region of steel plate where cooling rate is high is hardened more than the middle portion of the plate, thus HIC is generated near the surface of steel. Furthermore, microstructure of those high strength steel plates obtained by accelerated cooling is bainite or acicular ferrite, which is relatively high susceptible to cracking, not only in the near surface region but also in the middle region of the plate. Therefore, even if a countermeasure to HIC due to center segregation is applied, it is difficult for high strength steel plates of API X65 grade or higher to avoid completely the HIC originated from sulfide or oxide inclusions. Consequently, to improve HIC resistance of those high strength steel plates, a countermeasure to HIC due to sulfide or oxide inclusions is necessary.
On the other hand, as for high strength steels having excellent HIC resistance without massive bainite and martensite which are highly susceptible to cracking, JP-A-7-216500 discloses an API X80 grade high strength steel having excellent HIC resistance, which consists of ferrite and bainite. JP-A-61-227129 and JP-A-7-70697 disclose high strength steels whose SCC (SSCC) resistance and HIC resistance are improved by controlling microstructure to a single phase of ferrite and by utilizing carbide precipitation strengthening attained by adding large amount of Mo or Ti.
However, bainite phase in a ferrite-bainite steel consisting of ferrite and bainite described in JP-A-7-216500 has relatively high susceptibility to cracking, which is not so high as that of massive bainite or martensite. Accordingly, contents of S and of Mn are required to be strictly controlled, and Ca treatment is essentially required for improving HIC resistance, thus increasing manufacturing cost. Furthermore, the ferrite phase described in JP-A-61-227129 and JP-A-7-70697 has good ductility to show very low susceptibility to cracking, thus HIC resistance is significantly improved compared with bainite or acicular ferrite. The steel consisting of a single phase of ferrite, however, has low strength. Therefore, the steel described in JP-A-61-227129 contains large amount of C and Mo to precipitate large amount of carbides for increasing strength. In a steel strip described in JP-A-7-70697, the strength is improved by coiling a Ti-added steel strip at a specified temperature to utilize TiC precipitation strengthening. To obtain the ferritic microstructure containing dispersed Mo carbides, as described in JP-A-61-227129, however, it is necessary to apply cold working after quenching and tempering, and further to apply tempering, which raises manufacturing cost. In addition, coarse Mo carbides as large as about 0.1 μm are produced, resulting in small effect for strength increase. As a result, it is necessary to increase the amount of carbides by further addition of C and Mo for attaining desired strength. TiC used in a high strength steel described in JP-A-7-70697 is finer than MoC, and is effective for precipitation strengthening. However, the disclosure gives no countermeasure to growth of the TiC, which is easier to grow depending on precipitation temperature. As a result, precipitation strengthening is not sufficient, and large amount of Ti has to be added. In addition, the steel with addition of large amount of Ti shows significant deterioration of toughness of welding heat-affected zone.